In Pokémon Black 2 & White 2, a new feature called Hidden Grottoes were introduced. In these spots, besides random item spawns, players get to encounter Pokémon that have their Hidden Ability, which cannot be normally found in standard grass/cave/water encounters in the games. The levels varies, depending on which Grotto the Pokémon was encountered in. It is important to note that any kind of Hidden Grotto encounter cannot be shiny.
Besides the standard grotto encounters, the in-game mechanic Funfest Missions also allows for different Pokémon to be encountered, though all but one of such mission are local wireless missions.
This page hosts files of the various different species that can encountered in the standard grotto, as well as Funfest Mission grottoes.
To be completely transparent, the Funfest grotto encounters were obtained using RAM manipulation, as detailed here. The only reason why this is done and why the files are included, is because it appears the encounters generated are indistinguishable from the actual Funfest mission.
Note: PKHeX may flag Poliwag/Chansey/Togetic/Magneton/Dragonair/Shelgon as illegal, presumably because it's information wasn't added.

Once again, using my Wonder Card analogy, not entire true. They probably have a set of missions that they intend to do.
I mean, B2W2 has PokéPark 2020 text planned in it. Is there code for 2020 attendance? No, not really, it's just a description kept for easy access.
They just have a flag for missions, and when they have a local wireless beacon for it, rather than program in the whole text, just pull the in-game text entry for said flag. This way, at the very least everyone will have the instructions in a language they can read.
Flavor text doesn't have to be localized (think wonder card titles, wonder card full flavor text), but the key in-game text that is used for instructions has to be, so that people understand what to do. The easiest way to handle that is have it all programmed in the game, and simply get a flag to pull that text. (Like the Park example above)
To be honest I like this description, but past Gen 3, I don't think they've done any kind of mechanisms in the games that allow for new/extra subroutines, presumably cause they're afraid for ACE.
Plus, Quiz subroutines ARE in the game, they just need to change talking to clowns to talking to scientists.
Or simply check the RAM from pre-mission and during mission. The byte for "which mission was chosen" probably has to change ya? Then make it a code, activate a different mission, see if it still gave you perimeters for the 'code' instead.
I don't exactly have time to search through more lines in RAM, with school work coming up. I just did it last night since I was distracted and curious, but it doesn't seem like much more progress can be made.
I doubt I'll proceed any further than this, I am content with knowing how to change the encounters in RAM.
At least if we understood how the wonder card structure works, maybe we can gift ourselves permanently special missions, just maybe..
------ EDIT: I wrote a cheat code that modifies the Funfest Grotto Species
Press Select to Spawn 649 Masterballs into the first slot of your bag.
The species that will be encountered is based on the amount of balls in the first item slot.
Japanese B2W2
and for ENG B2W2

While I've not been able to find the location that controls/shows which mission is being played, I've found the location that modifies the special spawn list that is referenced for the regular hidden grotto funfest mission.
As much as I'll like to hope that the Special Funfest missions are in the game (the text are), they probably aren't.
If we think about if from a Wonder card's prospective:
1. Descriptions are in the game (which description displayed controlled by WC)
2. Subroutine on how to generate and gift a Pokémon is in the game
3. The species given isn't in the game (it's on wonder card) For all we know, it's the same case:
1. Description for missions are in the game (we know this)
2. Subroutine on how to generate the Hidden Grotto is in the game (they could just use the Noisy/Quiet subroutine)
3. The species encounter-able isn't in the game (we don't know this but who knows at this point...)
The first step anyone can take (besides this injection method), is to find Stunky/(forgotten name) in the garc.
edit: Apparently there may be a 4th wonder card mode. Not sure if it applies to special missions.
I can't seem to make it change the Funfest Title or control what I've 'received', tho.

Hey, please don't double post. Anything new and if no one has responded, just edit into your existing post. I've merged your 2 posts for now.
GBA versions of GS or AR have a GBA save dumping feature.
While the save may or may not be directly compatible with PKHeX, they can be imported into VBA and converted to .SAV
Upload the save. I'm expecting it to be corrupted though, cause that'll be the main reason it doesn't work on PKHeX
(More often than not, PKHeX issues are caused by the SAV, not PKHeX)

PKHeX works for Gen 3. To inject events into Gen 3, use this.
It appears your save is either encrypted or corrupted, hence why PKHeX won't open it.
Look at it. There's no empty space at all, looks like a garbled mess. And your save is like this the whole way.
Now look at mine. There's empty space. There's a pattern to the data. (note: I'm using different offset/location as starting location was pretty much empty.)
Redump your save, or use a different dumping method.
edit: Also, if there's nothing wrong with the dumping process, it may be possible your cart is a fake cart, hence all the issues.